Optical cantilever based analyte detection

ABSTRACT

An apparatus for detecting a deflection of a beam, the apparatus comprising a beam having a first side and a second side; and a grating structure positioned adjacent the second side of the beam, the grating structure including an interrogating grating coupler configured to direct light towards the beam; wherein the beam and the interrogating grating coupler form a resonant cavity, and light input to the resonant cavity is modulated according to the deflection of the beam.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/035,374, filed Feb. 25, 2011 and U.S. patent applicationSer. No. 13/761,987, filed Feb. 7, 2013, which claims priority toAustralian Patent Application No. 2012900444, filed Feb. 7, 2012, thedisclosures of which are hereby incorporated by reference in theirentirety.

FIELD OF THE INVENTION

The present invention relates to a system, an apparatus and a method fordetermining a deflection of a beam or more particularly the deflectionof a cantilever.

BACKGROUND OF THE INVENTION

Different methods for detecting chemical and biological analytes havebeen used. Such technology has been used, for example, in processcontrol, environmental monitoring, medical diagnostics and security.

Mass spectroscopy is one approach to detect such analytes. The processbegins with an ionized sample. The ionized sample is shot through avacuum that is subjected to an electromagnetic field. Theelectromagnetic field changes the path of lighter ions more than heavierions. A series of detectors or a photographic plate are then used tosort the ions depending on their mass. The output of this process, whichis the signal from the detectors or the photographic plate, can be usedto determine the composition of the analytes in the sample.

A disadvantage of mass spectroscopy instruments is that they aregenerally high-cost instruments. Additionally, they are difficult toruggedize, and are not useful for applications that require a sensorhead to be remote from signal-processing electronics.

A more recent approach is to use Micro Electro Mechanical Systems(MEMS)-based microstructures, and more specifically micro-cantilevers.These are extremely sensitive systems, and several demonstrations ofmass sensors that have detection limits as low 10⁻²¹ g, approximatelythe mass of a single protein molecule, have been performed. While theseexperiments have been performed in idealised environments, practicalcantilever-based systems have been demonstrated for the detection of awide range of single analytes.

A portion of the micro-cantilever is coated with an analyte selectivecoating to which the analyte is adsorbed.

There are two common modes of operation of micro-cantilever sensors,namely static and dynamic.

In the static mode, a stress differential is induced across thecantilever due to preferential adsorption of an analyte onto the analyteselective coating causing the cantilever to bend. The extent of thebending is in direct relation to the amount of analyte adsorbed. Thestress differential can be induced by the analyte causing swelling of anoverlayer, or by changes in the Gibbs free energy of the surface.

In the dynamic mode, the adsorbed analyte changes the mass of thecantilever and hence its mechanical resonance frequency. The rate andsize of the change in resonance frequency is then measured to estimatethe analyte concentration. Active sensing using these structures isachieved by resonant excitation.

In general, long, compliant cantilevers are required for sensitivestatic sensors, while high sensitivity for dynamic sensors dictate thatshort, stiff beams with high Q-factor mechanical resonances are needed.The most sensitive MEMS-based sensors to date have been based onmeasurements of resonant frequency.

Readout technologies used with micro-cantilever sensors are primarilybased on optical techniques developed for atomic force microscopy (AFM)analysis. Here, light is reflected from the cantilever tip to a distantquadrant detector, which process is referred to as optical leveraging.Electrical sensing and optical sensing techniques are also used.Electrical sensing includes piezoresistive, piezoelectric, capacitive,Lorentz force/emf sensing and tunnelling current techniques. Opticalsensing techniques include optical sensing based on opticalinterference, the optical interference being either in an interferometeror in the use of diffraction from an optical grating formed by a line ofcantilevers. This latter configuration using an optical grating formedby a line of cantilevers is often described as an array in literature,but is still effectively a sensor for a single analyte.

Another approach to analyte detection is where large, compact,integrated arrays of individual sensors are used, particularly formulti-analyte, multi-analysis applications. These are particularlyuseful when an unknown substance is to be identified or if there is anumber of chemical species to be tested for simultaneously. Examples ofsuch requirements can be found in the screening of food for pesticideresidues where there are many different potential contaminants,detection of different antibodies in a single blood sample, or thepresence of any of the many possible illicit drugs or explosives inluggage. Additionally, an array of sensors can also give significantlyimproved statistics of detection (including fewer false-positives andfalse-negatives) by averaging the response over a large number ofsensors, and allows the use of multivariate statistical chemometrictechniques, as are typically applied in spectroscopic analysis.

There are several disadvantages with the sensors of today. There is, forexample, a lack of compact, robust and cost-effective read-outtechnology that combines high sensitivity with high dynamic range.Sensors that are good at detecting small amounts of analyte typicallyhave poor dynamic range which is especially noticeable when the levelsof analyte are large. A problem with AFM-based cantilever systems isthat they are very large as they incorporate bulky free space opticsrequiring a sensor for each cantilever output. A problem with electricalcantilever systems is that they require extensive power on-chipelectronics.

As is known in the art, an Atomic Force Microscope (AFM) consists of acantilever with a pointed tip or probe at its end that is used to scan asample surface. The cantilever is typically made of silicon or siliconnitride with a tip radius of curvature in the order of nanometers usingmicro-electromechanical fabrication techniques. When the tip is broughtinto proximity of the sample surface, forces between the tip and thesample lead to a deflection of the cantilever according to Hooke's law.

Interatomic forces between the probe tip and the sample surface causethe cantilever to deflect as the sample's surface topography (or otherproperties) change as the tip is scanned across the sample. A laserlight reflected from the back of the cantilever measures the deflectionof the cantilever. This information is fed back to a computer, whichgenerates a map of topography and/or other properties of interest.

Various measurements can be made including measuring either thedeflection of the cantilever (static mode) or a vibration frequency ofthe cantilever (dynamic mode). In some applications, the tip is coatedwith a thin film of ferromagnetic material that reacts to magnetic areason the sample surface. Some applications include:

-   -   Measuring 3-dimensional topography of an integrated circuit        device    -   Roughness measurements for chemical mechanical polishing    -   Analysis of microscopic phase distribution in polymers    -   Mechanical and physical property measurements for thin films    -   Imaging magnetic domains on digital storage media    -   Imaging of submicron phases in metals    -   Defect imaging in IC failure analysis    -   Microscopic imaging of fragile biological samples    -   Metrology for compact disk stampers

A problem with current AFMs is that the sensitivity is limited by shotnoise in the optical detection system. Although Brownian motion of thecantilever is a contributor to the noise, in practice it is not a factoras the shot noise is substantially greater than the noise induced byBrownian motion. While noise induced by Brownian motion may be reducedby cooling the cantilever, this is not practical for current AFMs as itmay interfere with the alignment of the optical system. A furtherproblem is that, the process of measuring an entire surface of a sampleis time consuming, as the probe tip must make many passes over thesample in order to build up an image.

Yet a further problem with current AFMs is that the probe often needs tobe replaced, and each time the probe is replaced the optical detectionsystem needs to be re-calibrated, which is a time consuming process.

There is therefore a need for an improved system and method ofperforming atomic force measurements.

SUMMARY OF THE INVENTION

In a broad form, the invention resides in an apparatus for detecting adeflection of a beam, the apparatus comprising:

a beam having a first side and a second side; and

a grating structure positioned adjacent the second side of the beam, thegrating structure including an interrogating grating coupler configuredto direct light towards the beam;

wherein the beam and the interrogating grating coupler form a resonantcavity, and light input to the resonant cavity is modulated according tothe deflection of the beam.

In another broad form, the invention resides in a method for detecting adeflection of a beam, the method comprising the steps of:

inputting light into a resonant cavity formed between a beam and agrating structure of a sensor;

receiving light modulated by a deflection of the beam; and

analysing the modulated light to determine the deflection of the beam.

Preferably, the beam is a cantilever.

In one embodiment, the invention resides in an apparatus for detecting apresence of one or more analytes in a sample, the apparatus comprising afirst cantilever comprising an analyte selective coating that isselective to said one or more analytes, a first grating coupledresonating structure positioned adjacent to the cantilever andcomprising a first interrogating grating coupler, wherein the firstinterrogating grating coupler and the cantilever form an opticalresonant cavity.

Preferably, the cantilever is dynamic.

Alternatively, the cantilever is static.

Preferably, the apparatus further comprises a second grating coupledresonating structure wherein the second grating coupled resonatingstructure comprises a second interrogating grating coupler; and thesecond interrogating grating coupler and the cantilever form an opticalresonant cavity.

Preferably, the second grating coupled resonating structure ispositioned adjacent to the first grating coupled resonating structure onan axis substantially parallel to the cantilever.

Preferably, the apparatus further comprises a signal analyser fordetection of the presence of one or more analytes in the sample.

Preferably, the signal analyser compares light modulated by the firstgrating coupled resonating structure and the cantilever with a pluralityof predefined signals.

Preferably, the first grating coupled resonating structure provides aninitial measurement, and the second grating coupled resonating structureprovides a refinement of said initial measurement.

Preferably, the first grating coupled resonating structure and thesecond grating coupled resonating structure are used to determine ashape of the cantilever.

Optionally, the apparatus further comprises:

a second cantilever;

a second grating coupled resonating structure comprising a secondinterrogating grating coupler;

wherein the second interrogating grating coupler and the secondcantilever form an optical resonant cavity.

Preferably, the first grating coupled resonating structure and thesecond grating coupled resonating structure are optically coupled inseries.

Optionally, the first grating coupled resonating structure and thesecond grating coupled resonating structure are optically coupled inparallel.

In another form, the invention resides in a method of detecting thepresence of one or more analytes in a sample. The method comprises thesteps of applying the sample to a cantilever, wherein the cantilevercomprises an analyte selective coating selective to the one or moreanalytes, passing an optical signal through a grating coupled resonatingstructure, wherein the grating coupled resonating structure is arrangedto form a resonant cavity with the cantilever; and analysing the opticalsignal.

Preferably, the cantilever is dynamic, and the step of analyzing theoptical signal comprises determining the resonance frequency of thecantilever and comparing the resonance frequency to known resonantcharacteristics of the cantilever.

Alternatively, the cantilever is static, and the analysis step comprisesdetermining a deflection of the cantilever.

Preferably, the cantilever is dynamic, and the step of analyzing theoptical signal comprises determining the resonance frequency of thecantilever and comparing the resonance frequency to known resonantcharacteristics of the cantilever.

Preferably, the step of analyzing the optical signal comprises comparingthe optical signal to a plurality of predefined signals.

Preferably, the method further comprises the steps of passing a secondoptical signal through a second grating coupled resonating structure,wherein the second grating coupled resonating structure is arranged toform a resonant cavity with the cantilever, and analyzing the secondoptical signal.

Preferably, the step of analysing the optical signal comprisesestimating an initial cantilever deflection measurement, and the step ofanalyzing the second optical signal comprises refining the initialcantilever deflection measurement

Preferably, the method further comprises the step of estimating a shapeof said cantilever, wherein the step of analysing the optical signalcomprises estimating a cantilever deflection measurement at a firstposition, and the step of analysing the second optical signal comprisesestimating a cantilever deflection measurement at a second position.

In yet another form, the invention resides in a system for performingatomic force measurements including:

a sensor including:

a beam having a first side and a second side, the beam including a tippositioned on a surface of the first side for interacting with a sample;and

a grating structure positioned adjacent the second side of the beam, thegrating structure including an interrogating grating coupler configuredto direct light towards the beam;

a light source optically coupled to an input of the sensor for inputtinglight; and

an analyser coupled to an output of the sensor; wherein

the beam and the interrogating grating coupler form a resonant cavity, amovement of the beam modulates the light source and the analyserdetermines a deflection of the beam according to the modulated light.

Preferably, the beam is a cantilever beam. Alternatively, the beam isfixed at opposite ends and includes a flexible portion between the ends.Preferably, the tip is positioned between the two ends of the beam.

Preferably, the modulated light is amplitude modulated. Alternatively oradditionally, the modulated light is frequency modulated.

Preferably, the system includes a plurality of sensors.

Preferably, the system further includes a de-multiplexer wherein aninput of the de-multiplexer is optically connected to the light sourceand each output of a plurality of outputs of the de-multiplexer isoptically connected to a respective input of a grating structure of arespective sensor.

Preferably, the system further includes a multiplexer wherein eachoutput of the plurality of grating structures of a respective sensor isoptically connected to an input of the multiplexer, and the output ofthe multiplexer is connected to the analyser.

Preferably the de-multiplexer is a wavelength division de-multiplexer.

Preferably, light input into the multiplexer is separated into aplurality of discrete wavelengths and/or wavelength bands.

Preferably, each wavelength of the plurality of discrete wavelengths ismodulated by a respective sensor.

In yet another form the invention resides in a method of performingatomic force measurements on a sample, the method including the stepsof:

inputting light into a resonant cavity formed between a beam and agrating structure of a sensor;

receiving at an analyser light modulated by a movement of the beam; and

analysing the modulated light to determine a characteristic of thesample.

Preferably, the characteristic is a topography of the sample.

BRIEF DESCRIPTION OF THE DRAWINGS

To assist in understanding the invention and to enable a person skilledin the art to put the invention into practical effect, preferredembodiments of the invention will be described by way of example onlywith reference to the accompanying drawings, in which:

FIG. 1 shows a side sectional view of an optical microcantileverwaveguide, according to the prior art;

FIG. 2 shows a top perspective view of an optical microcantilever sensoraccording to an embodiment of the invention;

FIG. 3 shows a front sectional view of the optical microcantileversensor according to an embodiment of the invention;

FIG. 4 shows a side sectional view of an optical microcantilever sensoraccording to a second embodiment of the invention;

FIG. 5 is a graph showing the periodic nature of a signal of atransmission power according to an embodiment of the invention;

FIG. 6 shows a schematic diagram of an optical microcantilever sensoraccording to a third embodiment of the invention;

FIG. 7 shows a schematic diagram of an optical microcantilever sensoraccording to a fourth embodiment of the invention;

FIG. 8 shows a schematic diagram of an optical microcantilever sensoraccording to a fifth embodiment of the invention;

FIG. 9 shows a schematic diagram of an optical microcantilever sensor900 according to a sixth embodiment of the invention;

FIG. 10 is a bottom perspective view of a system for performing atomicforce measurements;

FIG. 11 is a cross-sectional end view of a sensor of FIG. 1;

FIG. 12 is a block diagram illustrating a system of an array of thesensors shown in FIGS. 10 and 11 for performing atomic forcemeasurements; and

FIG. 13 illustrates a method of performing atomic force measurements onan object, according to an embodiment of the present invention.

DETAILED DESCRIPTION

While the present invention is open to various modifications andalternative constructions, the example embodiments shown in the drawingswill be described herein in detail. It is to be understood, however,there is no intention to limit the invention to the particular exampleforms disclosed. On the contrary, it is intended that the inventioncover all modifications, equivalences and alternative constructionsfalling within the spirit and scope of the invention as expressed in theappended claims.

FIG. 1 shows a side sectional view of an optical microcantileverwaveguide 100, according to the prior art. The optical microcantileverwaveguide 100 comprises a fixed component 102 and a dynamic component104. The fixed component is attached to an insulator 108 such as forexample SiO₂ or Si3N₄. The insulator 108 is attached to a substrate 110such as for example a Si substrate. This layered structure allows forthe simple construction of the optical microcantilever waveguide 100through layering of the substrate 110, the insulator 108 and the opticalcantilever waveguide 100, and by then etching away an area of theinsulator 108 (and possibly also an area of the substrate 110) forming avoid 112 under the dynamic component 104 of the optical microcantileverwaveguide 100. The dynamic component 104 of the microcantileverwaveguide 100 is optically coupled to a fixed waveguide 106.

The dynamic component 104 is free to move above the void 112 in theinsulator 108. Upon adsorbtion of an analyte, the mass of the dynamiccomponent 104 of the optical microcantilever waveguide 100 changes. Thischange in mass results in a change of a resonance frequency of theoptical microcantilever waveguide 100.

Light enters at an end of the fixed component 102 of the opticalmicrocantilever waveguide 100 and propagates along the waveguide 100 tothe dynamic component 104. Light exits the dynamic component 104 in adirection towards the fixed waveguide 106.

In a dynamic mode, the light entering the fixed waveguide 106 isamplitude modulated as a result of a coupling loss between the dynamiccomponent 104 and the fixed waveguide 106 that is in close proximity tothe dynamic component 104, which loss occurs as the dynamic component104 vibrates. The light entering the fixed waveguide 106 is nominallymodulated at twice the vibration frequency of the dynamic component 104for symmetric vibration. Alternatively, in a static mode, the dynamiccomponent 104 of the optical microcantilever waveguide 100 may changeshape upon adsorbtion of an analyte. In this case the light entering thefixed waveguide 106 has an amplitude based upon the shape of the dynamiccomponent 104 of the optical microcantilever waveguide 100.

The light entering the fixed waveguide 106 is analysed to detect thepresence of an analyte on the optical microcantilever waveguide 100. Thelight may be compared to light with known characteristics, such as forexample light modulated due to the presence of an analyte.Alternatively, the resonance frequency or shape of the opticalmicrocantilever waveguide 100 may be estimated and compared topre-determined characteristics.

The present invention resides in an apparatus for detecting a presenceof one or more analytes in a sample. The apparatus comprises acantilever and a grating coupled resonating structure positionedadjacent to the cantilever. The cantilever comprises an analyteselective coating that is selective to the one or more analytes. Thegrating coupled resonating structure comprises an interrogating gratingcoupler which forms an optical resonant cavity with the cantilever.

An advantage of the present invention is the ability to economicallyhave a very large number of sensors on a small surface, enablingefficient detection on multiple analytes. Furthermore it does notrequire bulky free space optics or extensive power on-chip electronics.

FIG. 2 shows a top perspective view and FIG. 3 shows a front sectionalview of an optical microcantilever sensor 200 according to an embodimentof the invention. As shown in FIG. 2 and FIG. 3, the opticalmicrocantilever sensor 200 comprises a cantilever 205 and a gratingcoupled resonating structure 210. The grating coupled resonatingstructure 210 comprises an input grating coupler 215, an interrogatinggrating coupler 220 and an output grating coupler 225. The interrogatinggrating coupler 220 is placed directly under and adjacent to thecantilever 205. The cantilever 205 comprises an analyte selectivecoating and a reflective surface 207, where the reflective surface 207is opposite the interrogating grating coupler 220. However it should beappreciated that the cantilever 205 may be a beam, or a cantileverwithout an analyte selective coating. Thus the present invention may beused to detect a movement of any type of beam or cantilever.

The input grating coupler 215 is optically connected to theinterrogating grating coupler 220 and the interrogating grating coupler220 is optically connected to the output grating coupler 225. The outputgrating coupler 225 is optically connected to a signal analyser, forexample through an optical fibre.

Referring to FIG. 3, arrows illustrate the light path of the lightthrough the optical microcantilever sensor 200.

Light is coupled to the input grating coupler 215 from a light source,via an optical waveguide or an optical fibre, for example. The lightpropagates along the grating coupled resonating structure 210 to theinterrogating grating coupler 220 and out of the interrogating gratingcoupler 220 in a near perpendicular direction towards the cantilever205. The light then propagates along the grating coupled resonatingstructure 210 to the output grating coupler 225.

The cantilever 205 and interrogating grating coupler 220 form a resonantcavity such that the amount and/or frequency of light coupled to theoutput grating coupler 225 is a function of the separation of theinterrogating grating coupler 220 and the cantilever 205.

The light is output from the grating coupled resonating structure 210via the output grating coupler 225 so that it may be analysed in realtime or stored for analysis at a later time.

When a sample is applied to the cantilever 205, adsorbtion of an analytemay occur depending on the analyte selective coating and a compositionof the sample.

A pattern or shape of the interrogating grating coupler 220, for exampledimensions of grooves of the interrogating grating coupler 220,determines a modulation of light resonating between the interrogatinggrating coupler 220 and the cantilever 205. Additionally, a change indistance between the cantilever 205 and the interrogating gratingcoupler 220 causes a change in the modulation of the light output fromthe output grating coupler 225.

A change in mass of the cantilever 205 occurs upon adsorbtion of theanalyte. In a dynamic mode of operation, the change in mass results in achange in resonance frequency of the cantilever 205 which may becompared to when the analyte is not present. The resonance frequency ofthe cantilever can be determined at the output grating coupler 225through resonant excitation of the cantilever 205.

Alternatively, in a static mode of operation, the presence of an analytecauses a change in shape of the cantilever 205. The change in shape ofthe cantilever 205 causes a change in the distance between thecantilever 205 and the interrogating grating coupler 220 a and hencechange in the light at the output grating coupler 225.

The signal analyser, which indicates the presence and concentration ofthe analyte in the sample, uses analysis of the light to estimate theresonance frequency of the cantilever 205, or in the case of a staticcantilever the shape of the cantilever 205.

The resonance frequency of the cantilever 205 in dynamic mode operation,or the shape of the cantilever 205 in static mode, may be compared toknown characteristics of the cantilever 205 to determine whether ananalyte is present or not. Known characteristics of the cantilever 205include resonance frequency without the presence of an analyte,resonance frequencies with the presence of a particular amount ofanalyte or concentration, shape without the presence of an analyte,shapes with the presence of a particular amount of analyte orconcentration.

In an embodiment of the invention, the resonance frequency, height orposition need not be calculated or estimated explicitly for eachcantilever and measurement. Predefined signals of the cantilever at, forexample, different resonance frequencies, heights or positions may becompared directly to the signal in the analysis step.

FIG. 4 shows a side sectional view of an optical microcantilever sensor400 according to a second embodiment of the invention. The opticalmicrocantilever sensor 400 comprises a cantilever 405 and a first,second and third grating coupled resonating structure 410 a, 410 b and410 c, respectively, which are each specific examples of the gratingcoupled resonating structure 210 of FIG. 2. Similarly, the cantilever405 is a specific example of the cantilever 205 of FIG. 2.

The first grating coupled resonating structure 410 a, placed under adistal end of the cantilever 405, can be used to measure fine changes inshape or fine movements in the cantilever 405.

The second grating coupled resonating structure 410 b is positionedadjacent to the first grating coupled resonating structure 410 a on anaxis substantially parallel to the cantilever 405. The second gratingcoupled resonating structure 410 b, placed under a central part of thecantilever 405, can be used when larger change in shape or largermovements are to be measured, possibly in combination with the firstgrating coupled resonating structure 410 a. In this case the secondgrating coupled resonating structure 410 b provides a refinement of aninitial measurement of the first grating coupled resonating structure410 a.

The third grating coupled resonating structure 410 c is positionedadjacent to the second grating coupled resonating structure 410 b on anaxis substantially parallel to the cantilever 405. The third gratingcoupled resonating structure 410 c is placed under a proximal end of thecantilever 405 and can be used when larger change in shape or largermovements are to be measured, possibly in combination with the first andsecond grating coupled resonating structures 410 a and 410 b. In thiscase the second grating coupled resonating structure 410 b provides arefinement of the initial measurement of the first grating coupledresonating structure 410 a and the refinement provided by the secondgrating coupled resonating structure 410 b.

As would be readily understood by those skilled in the art, any numberof grating coupled resonating structures 410 may be placed under asingle cantilever, and at any position, without deviating from thepresent invention.

The exemplary embodiments illustrated in FIG. 2, FIG. 3 and FIG. 4 areapplicable to both static and dynamic cantilevers 205, 405, and in bothgaseous and aqueous environments. Furthermore, the grating coupledresonating structure 210, 410 a, 410 b, 410 c can be orientedarbitrarily with respect to the cantilever 205, 405, and the design ofthe cantilever 205, 405 can be decoupled from the design of the gratingcoupled resonating structure 210, 410 a, 410 b, 410 c. A furthervaluable capability of this approach is that the multiple gratingcoupled resonating structures 210, 410 a, 410 b, 410 c under the singlecantilever 205, 405, as described in FIG. 4, allows for the shape of thecantilever 205, 405 to be measured with greater precision.

Since an analyte can initially be adsorbed anywhere along the analyteselective coating of the cantilever 205, 405, a change in shape of thecantilever 205, 405 can be used as an early indication of the presenceof the analyte. Further, as is discussed further in FIG. 5, it may beadvantageous to have multiple grating coupled resonating structures toenhance a dynamic range of the optical microcantilever sensor 200, 300,400.

FIG. 5 is a graph 500 showing the periodic nature of a signal 530 of atransmission power 510 according to an embodiment of the invention, withrespect to a separation 520 between the cantilever 205, 405 and thegrating coupled resonating structure 210, 410 a, 410 b, 410 c. As can beseen in the figure, separations 0.5, 1.25, 2 and 2.75 micrometers, forexample, have similar transmission powers 510. This ambiguity canhowever be removed, while still maintaining high sensitivity, bymeasuring the displacement of the cantilever 205, 405 at multiplepositions. FIG. 4 illustrates an example where multiple grating coupledresonating structures 210, 410 a, 410 b, 410 c are placed under a singlecantilever. Such configurations allow for Vernier-like calculations tobe made.

FIG. 6 shows a schematic diagram of an optical microcantilever sensor600 according to a third embodiment of the invention. The opticalmicrocantilever sensor 600 comprises a wavelength divisionde-multiplexer 605, the wavelength division de-multiplexer 605comprising three optical outputs 610 a, 610 b, 610 c, three gratingcoupled resonating structures 615 a, 615 b, 615 c, a cantilever 620 anda wavelength division multiplexer 625.

An optical input is optically coupled to the wavelength divisionde-multiplexer 605. The wavelength division de-multiplexer 605 processeslight from the optical input and splits the light into a plurality ofsubsignals, each subsignal having a particular wavelength or pluralityof wavelengths. In this example, the wavelength division de-multiplexer605 has the three optical outputs 610 a, 610 b, 610 c, each carryinglight corresponding to a different wavelength or wavelength band.

The optical outputs 610 a, 610 b, 610 c are optically coupled to thegrating coupled resonating structures 615 a, 615 b, 615 c. The gratingcoupled resonating structures 615 a, 615 b, 615 c are similar to thegrating coupled resonating structures 210, 410 a, 410 b, 410 c. Eachgrating coupled resonating structure 615 a, 615 b, 615 c is connected inparallel and forms an optical resonance cavity with the cantilever 620.The wavelength division multiplexer 625 additively combines the lightoutput from grating coupled resonating structures 615 a, 615 b, 615 csuch that an output signal of the wavelength division multiplexer 625comprises a single light signal comprising multiple wavelengths.

Analysis of an individual grating coupled resonating structure 615 a,615 b, 615 c, may be performed by using pre-known characteristics of thegrating coupled resonating structure 615 a, 615 b, 615 c. Thesecharacteristics include, for example, a wavelength throughput of thegrating coupled resonating structure 615 a, 615 b, 615 c.

FIG. 7 shows a schematic diagram of an optical microcantilever sensor700 according to a fourth embodiment of the invention. The opticalmicrocantilever sensor 700 comprises two cantilevers 705 a, 705 b andtwo grating coupled resonating structures 710 a, 710 b.

The grating coupled resonating structures 710 a, 710 b form resonantcavities with the cantilevers 705 a, 705 b. The grating coupledresonating structure 710 a is optically coupled to the grating coupledresonating structure 710 b in series, i.e. an output of the firstgrating coupled resonating structures 710 a is connected in an input ofthe second grating coupled resonating structures 710 b.

Cantilever and grating coupled resonating structure pairs, for example705 a and 710 a, or 705 b and 710 b, may be analysed individually. Thisis advantageous as each pair may be sensitive to a different analyte.The analysis may be performed by using pre-known characteristics of thegrating coupled resonating structure 710 a, 710 b or the cantilever 705a, 705 b. These characteristics include, for example, a resonancefrequency of the cantilever 705 a, 705 b and a wavelength throughput ofthe grating coupled resonating structure 710 a, 710 b given a separationto the cantilever 705 a, 705 b.

FIG. 8 shows a schematic diagram of an optical microcantilever sensor800 according to a fifth embodiment of the invention.

The optical microcantilever sensor 800 comprises a wavelength divisionde-multiplexer 805, the wavelength division de-multiplexer 805comprising two optical outputs 810 a, 810 b, two grating coupledresonating structures 815 a, 815 b, two cantilevers 820 a, 820 b and awavelength division multiplexer 825.

An optical input is optically coupled to the wavelength divisionde-multiplexer 805. The wavelength division de-multiplexer 805 processeslight from the optical input and splits the light into a plurality ofsubsignals, each subsignal having a particular wavelength or pluralityof wavelengths. In this example, the wavelength division de-multiplexer805 has the two optical outputs 810 a, 810 b, each carrying lightcorresponding to a different wavelength or wavelength band.

The optical outputs 810 a, 810 b are optically coupled to the gratingcoupled resonating structures 815 a, 815 b respectively. The gratingcoupled resonating structures 815 a, 815 b are similar to the gratingcoupled resonating structures 210, 410 a, 410 b, 410 c, 615 a, 615 b.Each grating coupled resonating structure 815 a, 815 b forms an opticalresonance cavity with the cantilevers 820 a, 820 b, respectively. Thewavelength division multiplexer 825 additively combines the light outputfrom grating coupled resonating structures 815 a, 815 b such that anoutput signal of the wavelength division multiplexer 825 comprises asingle light signal comprising multiple wavelengths.

Analysis of an individual cantilever grating coupled resonatingstructure combination, for example 815 a/820 a or 815 b/820 b, which areconnected in parallel, may be performed by using pre-knowncharacteristics of the grating coupled resonating structure 815 a, 815 bor the cantilever 820 a, 820 b. These characteristics include, forexample, a resonance frequency of the cantilever 820 a, 820 b and awavelength throughput of the grating coupled resonating structure 815 a,815 b.

FIG. 9 shows a schematic diagram of an optical microcantilever sensor900 according to a sixth embodiment of the invention.

The optical microcantilever sensor 900 comprises a wavelength divisionde-multiplexer 905, the wavelength division de-multiplexer 905comprising two optical outputs 910 a, 910 b, four grating coupledresonating structures 915 a, 915 b, 915 c, 915 d, four cantilevers 920a, 920 b, 920 c, 920 d and a wavelength division multiplexer 925. Theoptical microcantilever sensor 900 is similar to the embodimentsdescribed in FIG. 7 and FIG. 8, except for that the cantilevers 920 a,920 b, 920 c, 920 d and grating coupled resonating structures 915 a, 915b, 915 c, 915 d are coupled in a series and parallel configuration.

The terms ‘series’ and ‘parallel’ are used in this specification. Seriesrefers to the case where an output of a first grating coupled resonatingstructure is optically connected to an input of a second grating coupledresonating structure. Parallel refers to the case where an input isshared between a first and second grating coupled resonating structure.Parallel connections include the case where the first grating coupledresonating structure uses or modifies a first part of the input, and thesecond grating coupled resonating structure uses or modifies a secondpart of the input, even where a series physical connection exists.

Additionally, as is understood by a person skilled in the art, anynumber of parallel and series connections may exist on a single sensor.

As will be understood by those having ordinary skill in the art, inlight of the present description, advantages of the present inventioninclude the ability to economically have a very large amount of sensorson a small surface, enabling efficient detection on multiple analytes.Furthermore, the detection of analytes with high precision and fidelityis possible. These efficient sensors may be used for the efficient andeconomical detection of pesticides or other chemicals in food, forefficient detection of explosives, narcotics or other elicit substancesjust to name a few examples.

FIG. 10 is a bottom perspective view of a system 100 for measuringAtomic Force according to a further embodiment of the present invention,and FIG. 11 is a cross-sectional end view through A-A of FIG. 10.Referring to FIGS. 10 and 11, the system 1100 includes a sensor 1200, alight source 1300 and an analyser 1400. The light source 1300 isconnected to an input of the sensor 1200 by an optical waveguide 1700Asuch as an optical fibre. An output of the sensor 1200 is connected tothe analyser 1400 by an optical waveguide 1700B such as an opticalfibre.

In one embodiment, the sensor 1200 is made usingmicro-electro-mechanical systems (MEMS) technology and includes a beamin the form of a cantilever beam 1210 and a grating structure 1220positioned adjacent the cantilever beam 1210. The cantilever beam 1210is planar and includes a tip 1211 which is pointed. The cantilever beam1210 includes a first side 1214 and a second side 1215. The tip 1211 ispositioned on the first side 1214 of the cantilever beam 1210 andtowards a distal end 1213 of the cantilever beam 1210. The tip 1211extends away from the cantilever beam 1210 towards a sample 1800 to bemeasured. A proximal end 1212 of the cantilever beam 1210 is fixedallowing the distal end 1213 to flex as the tip 1211 is moved over thesample 1800.

In another embodiment (not shown), the beam is fixed at both the distalend and the proximal end but allowing the beam to flex. A tip ispositioned in between the distal end and the proximal end. Preferablythe tip is positioned mid-way between the distal end and the proximalend. However it should be appreciated that the tip may be positionedanywhere between the distal end and the proximal end. By fixing the beamat both ends, Brownian motion is reduced, and sensitivity of ameasurement is increased.

In one embodiment, the grating structure 1220 uses Silicon on Insulator(SOI) technology and includes a substrate 1221, a buried oxide layer1222 and a waveguide layer 1223. Furthermore, the substrate 1221 and thewaveguide layer 1223 are made from silicon. The buried oxide layer 1222is formed on the substrate 1221 and the waveguide layer 1223 is formedon the buried oxide layer 1222. The waveguide layer 1223 includesgrooves to form an interrogating grating coupler 1224 and theinterrogating grating coupler 1224 is positioned adjacent the secondside 1215 of the cantilever beam 1210. The interrogating grating coupler1224 is similar to the interrogating grating coupler 220 shown in FIGS.2 and 3. In one embodiment, the waveguide layer 1223 is 220 nm thickfabricated over a 2000 nm buried oxide layer 1222 using an infra-redlight source 1300. However it should be appreciated that otherthicknesses may be used, for example between 100 nm and 2000 nm.

Although the grating structure 1220 has been described in relation toSOI technology, a person skilled in the art will appreciate that thegrating structure 1220, including the waveguide layer 1223 and theburied oxide layer 1222, may be made from many other materials. The mainrequirement is that the waveguide layer 1223 has a higher refractiveindex than the buried oxide layer 1222 so as to get the total internalreflections in the waveguide. For example, the waveguide layer 1223 mayalso be made from, but is not limited to, Germanium (Ge) and Silicon OxyNitride and the buried oxide layer 222 may be made from SU-8, Silicondioxide (SiO2) or Magnesium Oxide (MgO). In addition, the thicknessesused to fabricate the waveguide layer 1223 and the buried oxide layer1222 depend on the materials used and the wavelength of the light source1300.

Typically a gap between the interrogating grating coupler 1224 and thecantilever beam 1210 is between 0.05 and 10 μm. However it should beappreciated that other distances may be used depending on the wavelengthof the light source 1300 and the types of materials used for the gratingstructure 1220.

Although in the example above the sensor 1200 has been designed using alight source 1300 in the infra red band (with a wavelength of between0.74 μm to 30 μm), it should be appreciated that the light source 1300may produce light in the visible band (with a wavelength between 390 nmto 750 nm) or the ultra-violet band (with a wavelength between 10 nm to400 nm).

A pattern or shape of the interrogating grating coupler 1224, forexample dimensions of grooves of the interrogating grating coupler 1224,determines a resonance of light resonating between the interrogatinggrating coupler 1224 and the cantilever beam 1210.

In use, the unmodulated light 1500 is input to the sensor 1200. Theunmodulated light 500 propagates along the silicon waveguide layer 223until the unmodulated light 1500 exits the waveguide layer 1223 towardsthe cantilever beam 1210 at the interrogating grating coupler 1224. Theinterrogating grating coupler 1224 couples and directs light out of thewaveguide 1223 towards the cantilever beam 1210 and couples lightreflected from the second side 1215 of the cantilever beam 1210 backinto the waveguide thereby forming a resonant cavity with the cantileverbeam 1210. As the cantilever beam 1210 moves towards and away from theinterrogating grating coupler 1224, an intensity and/or frequency oflight output to the analyser 1400 is modulated as a function of theseparation between the interrogating grating coupler 1224 and thecantilever beam 1210. From the modulation, the analyser 1400 maydetermine a displacement of the cantilever beam 1210 in order, forexample, to determine a topography. In some embodiments the second side1215 of the cantilever beam 1210, is coated with a reflective materialsuch as gold in order to increase the reflectivity.

Although referred to as unmodulated light, it should be appreciated thatthe light input to the sensor 1200 may be modulated with a firstmodulation. As the first modulated light passes through the sensor 1200it is modulated by a second modulation. The second modulation may thenbe analysed by the analyser 1400 in order to determine a displacement ofthe cantilever beam 1210.

It should be appreciated that in order to perform a scan, the sample1800 may be fixed and the sensor 1200 moved across the sample 1300 underthe control of the analyser 1400. Alternatively, the sample 1800 may bemoved under the control of the analyser 1400 and the sensor 1200 may bestationary.

An electrostatic element may be used to control an initial deflection ofthe beam so as to tune the resonance of the optical cavity to its mostsensitive position. An electrode is placed underneath the beam orcantilever beam 1210, but not over the grating structure 1220. Thevoltage between the electrode and the metal on the underside of the beamis then controlled to attract or repel the beam as necessary.

The sensor 1200 of the present invention may be used in an array inorder to measure larger sections of the sample 1800. FIG. 12 is a blockdiagram illustrating a system of an array of sensors 2100 shown in FIGS.10 and 11 for performing atomic force measurements according to anembodiment of the present invention. The system 1900 includes aplurality of sensors 1200A, 1200B, 1200C formed in a row. However itshould be appreciated that the sensors 1200A, 1200B, 1200C may bepositioned in any suitable arrangement.

In this embodiment, the light source 1300 is connected to a wavelengthdivision de-multiplexer 1910 via a single optical waveguide 1700A. Thewavelength division de-multiplexer 1910 separates the light source 1300into a plurality of discrete wavelengths or wavelength bands λ₁, λ₂ andλ₃. Each output of the wavelength division de-multiplexer 1910 isconnected to a respective sensor 1200A, 1200B, 1200C by a respectiveoptical waveguide 1700C, 1700D, 1700E in order to couple the light ateach wavelength λ₁, λ₂, λ₃ to a respective grating structure 1220 of arespective cantilever sensor 1200A, 1200B, 1200C. As each cantileverbeam 1210 of a respective sensor 1200A, 1200B, 1200C moves it modulatesthe light at the respective wavelength or wavelength band λ₁, λ₂, λ₃.

The modulated light 1600 at each wavelength λ₁, λ₂, λ₃ is thenmultiplexed by a multiplexer 1920. Each sensor 1200A, 1200B, 1200C isconnected to the multiplexer 1920 by a respective optical waveguide1700F, 1700G, 1700H such as an optical fibre. An output of themultiplexer 1920 is connected to the analyser 1400 by optical waveguide1700B and the modulated light at each wavelength λ₁, λ₂, λ₃ is passed tothe analyser 1400. The analyser 1400 analyses the modulated light 1600at each discrete wavelength or wavelength band λ₁, λ₂, λ₃ to determine amovement of each sensor 1200A, 1200B, 1200C and accordingly determine acharacteristic of the sample 1800.

In another embodiment, the light from the light source 1300 may not bede-multiplexed into separate wavelengths; rather each sensor 1200 in thearray may be supplied from its own light source 1300 or with a samewavelength of light from a same light source 1300. Furthermore, anoutput from each sensor 1200 may connect to a separate analyser 400, andeach output analysed using a computer for example.

According to certain embodiments, the system 1100 includes a movementsensor (not shown), to determine the relative motion between the sensor1200 and the sample 1800. This enables the determination of a contour ofa sample irrespective of the rate of movement of the sample.

FIG. 13 illustrates a method 2000 of performing atomic forcemeasurements on an object, according to an embodiment of the presentinvention.

At step 2005, light is input into a resonant cavity formed between abeam and a grating structure of the sensor. A tip of the beam ispositioned adjacent to and in contact with the object, such that thebeam moves according to a contour of the sample.

At step 2010, the light from the resonant cavity is received at ananalyser, the light modulated according to a position of the beam.

At step 2015, the modulated light is analysed to determine a contour ofthe sample.

Steps 2005-2015 are advantageously performed on multiple points of theobject, either sequentially, for example through movement of the beamacross the object, in parallel, for example through the use of severalbeams and resonance cavities, or through a combination of series andparallel.

It should be appreciated that the present invention may be used in avariety of modes such as a static mode (where the beam flexes) and adynamic mode (where the cantilever beam oscillates) in order to performa variety of measurements.

For example the invention may be used in a contact mode where the sensoris scanned at a constant force between the sensor and a sample surfaceto obtain a 3D topographical map.

In an Intermittent Contact (Tapping Mode) the cantilever beam isoscillated at or near its resonant frequency. The oscillating tip isthen scanned at a height where it barely touches or “taps” the samplesurface. The analyser monitors the sensor position and a vibrationalamplitude to obtain topographical and other property informationallowing topographical information can be obtained even for fragilesurfaces.

An advantage of the present invention is that the optical readout of thegrating structure 1220 leads to increased sensitivity over existing freespace optical monitoring. The present invention uses an optical resonantcavity formed between the grating structure 1220 and the cantilever beam1210, or doubly clamped beam, coupled to a waveguide to increase anamplitude of a signal output from the sensor 1200 to levelssignificantly above the shot noise and thereby increasing the signal tonoise ratio.

Another advantage is that the necessity to align the optics of an AFMwhenever the probe is replaced is effectively eliminated due to theclose coupling of the optical cavity to the waveguide. This is becausethe sensor 1200 and the AFM may be fabricated such that when installed,the waveguide layer 1223 aligns with the light source 1300 in the AFM.

In addition Brownian motion noise may be reduced by clamping the beam ateach end and a further reduction in Brownian noise may be made bycooling the sensor 1200.

Finally, miniaturization of the AFM may be achieved allowing multiplebeams and AFM tips to form an array and to be integrated in the onestructure, effectively increasing the scan rate.

The above description of various embodiments of the present invention isprovided for purposes of description to one of ordinary skill in therelated art. It is not intended to be exhaustive or to limit theinvention to a single disclosed embodiment. As mentioned above, numerousalternatives and variations to the present invention will be apparent tothose skilled in the art of the above teaching. Accordingly, while somealternative embodiments have been discussed specifically, otherembodiments will be apparent or relatively easily developed by those ofordinary skill in the art. Accordingly, this patent specification isintended to embrace all alternatives, modifications and variations ofthe present invention that have been discussed herein, and otherembodiments that fall within the spirit and scope of the above describedinvention.

Limitations in the patent claims should be interpreted broadly based onthe language used in the claims, and such limitations should not belimited to specific examples described herein. In this specification,the terminology “present invention” is used as a reference to one ormore aspects within the present disclosure. The terminology “presentinvention” should not be improperly interpreted as an identification ofcritical elements, should not be improperly interpreted as applying toall aspects and embodiments, and should not be improperly interpreted aslimiting the scope of any patent claims.

What is claimed is:
 1. An apparatus for detecting a deflection of abeam, the apparatus comprising: a sensor including: a beam having afirst side and a second side; and a grating structure positionedadjacent the second side of the beam, the grating structure including aninterrogating grating coupler configured to direct light towards thebeam and couple light reflected from the beam such that the beam and theinterrogating grating coupler form an optical resonant cavity andmodulate input light in the resonant cavity according to deflection ofthe beam.
 2. The apparatus of claim 1 wherein the beam is a cantileverbeam.
 3. The apparatus of claim 2 wherein the cantilever beam includesan analyte selective coating for detecting one or more analytes in asample.
 4. The apparatus of claim 1 wherein the first side of the beamincludes a tip for interacting with a sample.
 5. The apparatus of claim1 wherein an input of the sensor is connected to a light source.
 6. Theapparatus of claim 1 wherein an output of the sensor is connected to ananalyser for analysing light modulated by the sensor.
 7. The apparatusof claim 1 wherein the beam is static.
 8. The apparatus of claim 1wherein the beam is dynamic.
 9. The apparatus of claim 1 wherein thebeam is fixed at one end.
 10. The apparatus of claim 1 wherein the beamis fixed at two opposed ends.
 11. The apparatus of claim 10 wherein thebeam includes a flexible portion between the opposite ends of the beam.12. The apparatus of claim 1 including a plurality of sensors.
 13. Theapparatus of claim 12 wherein an input of each of the plurality ofsensors is connected to an output of a de-multiplexer.
 14. The apparatusof the 12 wherein an output of each of the plurality of sensors inconnected to an input of a multiplexer.
 15. The apparatus of claim 13wherein an input of the de-multiplexer is connected to a light source.16. The apparatus of claim 14 wherein an output of the multiplexer isconnected to an analyser.
 17. The apparatus of claim 1 wherein thesensor includes a waveguide layer, and the interrogating grating coupleris formed in the waveguide layer.
 18. The apparatus of claim 17 whereinlight is input to, and output from, the sensor via the waveguide layer.19. A method for detecting a deflection of a beam, the method comprisingthe steps of: inputting light into a grating structure of a sensor;directing light from the grating structure into an optical resonantcavity formed between a beam and the grating structure; modulating thelight in the resonant cavity by deflection of the beam; receiving themodulated light through the grating structure; and analysing modulatedlight from the grating structure to determine the deflection of thebeam.
 20. The method of claim 19 wherein the beam is a cantilever.